feat: migrate iOS/macOS plugins and examples to Swift Package Manager

[vicajilau]

Summary

This PR migrates the iOS/macOS plugin stack and example projects to a coherent Swift Package Manager (SPM) setup, and also includes repository cleanup to remove generated noise and stale CocoaPods artifacts.

Closes #355

Why merge this

• Resolves an inconsistent hybrid state (SPM partially enabled while CocoaPods hooks still existed in iOS/macOS examples).
• Removes brittle Pod-related checks and references that caused intermittent build failures.
• Aligns plugin authoring layout with current Flutter SPM guidance.
• Improves repository hygiene by ignoring SPM build caches and removing tracked machine/generated files that should not live in git.
• Verified with successful iOS/macOS example builds after migration.

Main functional changes

1. Plugin SPM authoring updates

• Added Package.swift at the correct location.
• Removed misplaced open_file_ios/ios/open_file_ios/Sources/Package.swift.
• Updated Package.swift to use FlutterFramework and macOS 10.15 baseline.

2. Environment constraints for SPM-era Flutter tooling

• Updated pubspec.yaml and pubspec.yaml:
  • sdk: ^3.11.0
  • flutter: ">=3.41.0"

3. iOS example de-integration from CocoaPods remnants

• Deleted open_file_ios/example/ios/Podfile and open_file_ios/example/ios/Podfile.lock.
• Removed stale Pods refs/build phases/config ties from project.pbxproj.
• Removed Pods/Pods.xcodeproj from contents.xcworkspacedata.

4. macOS example workspace cleanup

• Removed stale Pods/Pods.xcodeproj reference from contents.xcworkspacedata.

Repository cleanup included in this PR

1. Git ignore hygiene

• Updated /.gitignore with recursive SPM cache ignores:
  • **/.build/
  • **/.swiftpm/
• Kept broader Flutter generated-file ignore coverage to reduce flutter pub get / build noise in a multi-package workspace.

2. Tracked generated/local files cleanup

• Removed tracked local CocoaPods files from iOS example (Podfile and Podfile.lock) as part of SPM migration.
• Removed tracked misplaced SPM manifest under Sources/.
• Previously cleaned local machine-specific file tracking policy (e.g. android local.properties in examples) is now consistent with ignore rules.

3. Lockfile normalization due to dependency refresh

• Updated:
  • pubspec.lock
  • pubspec.lock

iOS 13 note

• iOS minimum target effectively moved to iOS 13 in the SPM path.
• This came from Flutter migration behavior plus current SPM baseline guidance.
• This PR prioritizes a consistent, working SPM-first flow for iOS/macOS plugin examples.

Validation performed

• flutter pub get in platform packages.
• flutter build ios --simulator --no-codesign in example.
• flutter build macos in example.

Risk / rollback

• Primary behavior change is iOS 13 baseline in the SPM flow.
• If compatibility concerns arise, rollback is straightforward by reverting this PR.